How to properly aim at center of the screen using blend tree

Hello,

I'm working on a Third Person Shooter and I'm using a blend tree with different animation (aim up and aim down) and I wonder how can I compute the blend value in order to make the animation blending match with my character aiming at the target which is in front of him please. My camera is always behind the character and the forward of the character is always looking at the camera. All I want is my up and down animation blend tree output make that my character aim at the target.

You can find an fast illustration in the attachment files.

Thanks.